1. Standard Cosmology

2. Cosmological Red-Shift
Not really a Doppler effect
Space itself is being stretched between galaxies

3. Conclusions from our Observations
The Universe has a finite age, so light from very distant galaxies has not had time to reach us, therefore the night sky is dark.
The universe expands now, so looking back in time it actually shrinks until…?
Big Bang model: The universe is born out of a hot dense medium
13.7 billion years ago.

4. Big Bang The “start” of the universe, a primordial fireball
 the early universe was very hot and dense
 intimate connection between
cosmology and nuclear/particle
physics
“To understand the very big
we have to understand the
very small”
The study of the universe as a whole

5. How does the expansion work?
Like an explosion (hot, dense matter in the beginning), but space itself expands!
Slowed down by gravitational attraction
Attraction is the stronger, the more mass there is in the universe
Scientifically described by Einstein’s
General theory of Relativity (1915)

6. More General than Special Relativity
General Relativity is more general in the sense that we drop the restriction that an observer not be accelerated
The claim is that you cannot decide whether you are in a gravitational field, or just an accelerated observer
The Einstein field equations describe the geometric properties of spacetime

7. The Idea behind General Relativity
We view space and time as a whole, we call it four-dimensional space-time.
It has an unusual geometry, as we have seen
Space-time is warped by the presence of masses like the sun, so “Mass tells space how to bend”
Objects (like planets) travel in “straight” lines through this curved space (we see this as orbits), so
“Space tells matter how to move”

8. Planetary Orbits
Sun
Planet’s orbit

9. Effects of General Relativity
Bending of starlight by the Sun's gravitational field (and other gravitational lensing effects)

10. Assumption: Cosmological Principle
The Cosmological Principle: on very large scales (1000 Mpc and up) the universe is homogeneous and isotropic
Reasonably well-supported by observation
Means the universe has no edge and no center – the ultimate Copernican principle!
The study of the universe as a whole 10

11. What General Relativity tells us
The more mass there is in the universe, the more “braking” of expansion there is
So the game is:
Mass vs. Expansion
And we can even calculate who wins!

12. The Fate of the Universe – determined by a single number!
Critical density is the density required to just barely stop the expansion
We’ll use 0 = actual density/critical density:
0 = 1 means it’s a tie
0 > 1 means the universe will recollapse (Big Crunch)  Mass wins!
0 < 1 means gravity not strong enough to halt the expansion  Expansion wins!
And the number is: Ω0 = /- 0.02

13. The “size” of the Universe – depends on time!
Expansion
wins!
It’s a tie!
Mass wins!
Time

14. The Shape of the Universe
                                                 
In the basic scenario there is a simple relation between the density and the shape of space-time:
Density Curvature 2-D example Universe Time & Space
0>1 positive sphere closed, bound finite
0=1 zero (flat) plane open, marginal infinite
0<1 negative saddle open, unbound infinite

15. Back to: Expansion of the Universe
Either it grows forever
Or it comes to a standstill
Or it falls back and collapses (“Big crunch”)
In any case: Expansion slows down!
Surprise of the year 1998
(Birthday of Dark Energy):
All wrong! It accelerates!

16. Enter: The Cosmological Constant
Usually denoted 0, it represents a uniform pressure which either helps or retards the expansion (depending on its sign)
Physical origin of 0 is unclear
Einstein’s biggest blunder – or not !
Appears to be small but not quite zero!
Particle Physics’ biggest failure

17. Triple evidence for Dark Energy
Supernova data
Large scale structure of the cosmos
Microwave background
                                                                                              

18. Microwave Background: Signal from the Big Bang
Heat from the Big Bang should still be around, although red-shifted by the subsequent expansion
Predicted to be a blackbody spectrum with a characteristic temperature of Kelvin by George Gamow (1948)
 Cosmic Microwave Background Radiation (CMB)
PW Nobel in 1978; Dicke alerted them to the meaning

19. Discovery of Cosmic Microwave Background Radiation (CMB)
Penzias and Wilson (1964)
Tried to “debug” their horn antenna
Couldn’t get rid of “background noise”
 Signal from Big Bang
Very, very isotropic (1 part in 100,000)

20. CMB: Here’s how it looks like!
Peak as expected from 3 Kelvin warm object
Shape as expected from black body

21. CMB measurements improve

22. Latest Results: PLANCK
Measure fluctuations in microwave background
Expect typical size of fluctuation of ½ degree if universe is flat
Result:
Universe is flat !

23. Experiment and Theory
Expect “accoustic peak” at l=200
 There it is!

24. Supernova Data Type Ia Supernovae are standard candles
Can calculate distance
from brightness
Can measure redshift
General relativity gives us distance as a
function of redshift for a given universe
Supernovae are further away than
expected for any decelerating (“standard”)
universe

25. Pie in the Sky: Content of the Universe5%
Dark Energy
Dark Matter
SM Matter
23%
72%
We know almost everything about almost nothing!

26. Properties of Dark Energy
Should be able to explain acceleration of cosmic expansion  acts like a negative pressure
Must not mess up structure formation or nucleosynthesis
Does not dilute as the universe expands  will be different % of content of universe as time goes by